New Type of Galaxy Discovered

Vestiges of the original building blocks of the universe?

An international team of astronomers headed by Dr. Michael Drinkwater (Queensland, Australia), Dr. Michael Gregg (Livermore, USA) and Dr. Michael Hilker from the University of Bonn has discovered a new kind of small, very compact galaxy. Their findings are published in current issue of the prestigious academic journal Nature. This new type of galaxy could help to explain the discrepancy between observations and cosmological models.

The galaxies in our universe are not all the same: there are spirals which resemble our own Milky Way, large ellipticals and small low-luminosity galaxies. Astronomers have always wondered whether some types of galaxies may have been overlooked in previous observations. By making a large-scale spectroscopic checkout of all objects in the relatively close Fornax galaxy cluster the team of astronomers were now actually able to track down a new type: what is known as the ultra-compact galaxies lying in the centre of glaxy clusters. In appearance they resemble the galactic spherical diffuse nebula, but are many times bigger and brighter. Because of their very small extension in the sky the ultra-compact galaxies had previously been considered to be stars of the Milky Way. However, their radial velocity - a coarse way of measuring distance - revealed to the astronomers that they could not be in our Milky Way.

Proof thanks to Hubble\'s \'keen eye\'

In order to study the characteristics of the new type of galaxy the astronomers made use of state-of-the-art telescopes. The Hubble Space Telescope\'s \'keen eye\' was able to provide a spatial resolution of the ultra-compact galaxies, so that the team were in a position to determine their extent. Moreover, by using the big 8-metre telescopes of the European Southern Observatory (ESO) at El Paranal in Chile, Dr. Michael Hilker managed to measure spectroscopically how fast the stars move in the galaxy. Combining both measurements enabled the researchers to \'weigh\' the galaxies. The result was stunning: the extent, luminosity and mass are not matched in this combination by any other type of galaxy hitherto known.

This discovery could answer one question which has always given astronomers sleepless nights: according to received opinion the original building blocks of the universe are small dwarf galaxies of low mass, many of which must still be in existence even today. However, up to the present astronomers have always found far fewer dwarf galaxies than the cosmological models predicted. The researchers now believe that the ultra-compact galaxies may have originated from dwarf galaxies which have lost their peripheral stars. Computer simulations by Dr. Kenji Bekki (Sydney, Australia) also confirm that this is possible. The new type of galaxy is thus an important link which might explain the discrepancy between observations and cosmological models.

To download a high-resolution version of the picture, please click on the thumbnail. If you use the picture in your publication, we ask you to mention the author of the photo Dr. Michael Hilker.This picture shows in the background the Fornax galactic cluster, which is dominated by large elliptical galaxies. The enlargements used show an elliptical dwarf galaxy with core (top left) and one of the newly discovered ultra-compact galaxies (bottom right). The distribution of points demonstrates a simulation showing how a dwarf ellipse can lose the stars on its periphery by tidal energy while orbiting round the central galaxy, thereby becoming an ultra-compact galaxy.